The human carcinoembryonic antigen (CEA, CEACAM5, CD66e) is a glycosyl phosphatidyl (GPI)-linked glycoprotein that was originally described as a gastrointestinal oncofetal antigen [Gold and Freedman, 1985]. This cell surface antigen is frequently over-expressed on epithelial carcinomas of the intestinal and respiratory tracts, as well as cancers of the breast, pancreas, stomach, and ovary [Goldenberg at al., 1976; Shively at al., 1985; Thompson et al., 1991; Gold and Goldenberg, 1997; Hammarstrom, 1999]. From a clinical perspective, high preoperative levels of CEA in the blood of cancer patients negatively correlate with disease free survival. Intercellular adhesion events involving CEA have been linked to cancer invasion and metastasis [Jessup and Thomas, 1998; Yoshioka at al., 1998; Thomas at al., 1995]. As such, strategies interfering with CEA-specific functions and CEA-dependent cellular interactions may block or delay the establishment of metastatic tumour foci in vivo.
Structurally, CEA is composed of seven extracellular Ig-like domains (N, A1, B1, A2, B2, A3 and B3) and self-associates (defined as homotypic binding and homophilic cellular interactions) mainly through interactions involving its N and A3B3 Ig-like modules [Zhou et al., 1993]. Experimentally, the addition of monoclonal antibodies (mAbs) directed at epitopes found in the N domain of CEA [Jessup et al., 1993; Yamanka et al., 1996] as well as cyclic peptides derived from sequences within the N domain of CEA [Taheri et al., 2000] have been shown to inhibit CEA-specific cellular adhesion events in vitro. Similarly, administration of Fab′ recognizing epitopes located in the N and adjacent A1B1 domains of CEA have been shown to increase the survival of nude mice harbouring CEA-expressing lung micrometastases [Blumenthal et al., 2005]. These findings suggest that an immune response specifically focused at blocking interactions involving the N domain of CEA may halt or limit the formation of tumour metastases in patients.
Previous attempts at developing CEA-based anti-tumor vaccines have centered on vaccine formulations based either on dendritic cells preloaded with predicted T-cell epitopes or recombinant viruses delivering the full length molecule [Curigliano et al., 2006; Berinstein, 2002; Zimmer and Thomas, 2001; Crosti et al., 2006; Shen et al., 2004; Kobayashi et al. 2002; Matsuda et al., 2004]. The majority of putative T-cell epitopes have been to short sequences located in the central region of this molecule [Curigliano et al., 2006; Berinstein, 2002; Zimmer and Thomas, 2001; Crosti et al., 2006; Shen et al., 2004; Kobayashi et al. 2002; Matsuda et al., 2004]. In another instance, predicted T cell (CTL) epitopes were altered to include a Val residue as the last residue, as an attempt to improve the peptide binding to HLA-A2 and therefore mount CEA-specific CTL responses [WO2009002418]. Unfortunately, the lack of immunogenicity of these epitopes coupled with the presence of immuno-suppressive regulatory T (Treg) cells in tumour microenvironments were shown to compromise the efficacy of anti-tumour CEA-based vaccines [Morse et al., 2008; Bos et al., 2008]. Overcoming these limitations has been attempted either through the depletion of immuno-suppressive Treg cells [Morse et al., 2008; Bos et al., 2008] or by co-administering TAA in combination with co-stimulatory molecules [Gulley et al., 2008; Dai et al., 2008].
A therapeutic vaccine aimed at blocking CEA-dependent adhesion events and the establishment of tumour foci may represent a more appropriate and achievable objective. Importantly, the role of CEA in metastasis is linked to its over-expression and associations which correlates with the early inactivation of caspase-9 and activation of the PI3-K/Akt survival pathway as well as the inactivation of caspase-8 [Camacho-Leal and Stanners, 2008] presumably by directly binding tumor necrosis factor-related apoptosis-inducing ligand (TRAIL-R2) (DR5) through its PELPK motif (residues 108-112 of the N domain of CEA) [Samara et al, 2007]. This peptide motif is responsible for mediating the lodging of metastasizing cells to the hepatic parenchyma leading to the development of metastatic foci by promoting intercellular aggregations through homophile cell interactions involving the IgV-like N- and the IgC-like A3 domains [Berinstein, 2002; Benchimol at al., 1989; Taheri at al, 2000; Zimmer and Thomas, 2001].